The apparatus and process of the present invention relate to filtration of fluids, and more specifically to filtering or straining of fluids where the suspended solids removed by filtration must eventually be discharged from the filter as "dry cake" and not as a slurry. More particularly, the apparatus and process of the present invention relate to filtering of suspended contaminants in a fluid utilizing pre-coat filtration and where the suspended solids are considered to be "hazardous wastes".
When producing oil wells, it is a common practice to pump water downhole in order to force the oil out of a formation.
Other oil producing processes require large quantities of water to be utilized, with the result of contamination of the water with oil particles and other chemicals utilized in the process and impossibility of disposing of it by simply returning it to the land formation or by draining it back into the sea in applications at offshore platforms.
Still other processes relating to the oil and gas industry and also many other industries such as the Chemical and Petro-Chemical Industries produce contaminants which are removed from a liquid stream by filtration. Many of these contaminants are considered to be "hazardous wastes" and must be rendered to a dry form for proper disposal. Therefore, it is necessary that these contaminants be discharged from the filter as "dry cake" and not in the form of a slurry wet cake as when backflushing with a fluid. Dry cake discharge requires complete removal of the liquid from the filter interior.
To overcome the problem and withdraw contaminants from the used water, various filtration methods have been used. One of the methods involves the use of a layer of filter media, which normally comprises diatomaceous earth or perlite. An example of such method is disclosed in U.S. Pat. No. 4,704,210 issued on Nov. 3, 1987 and entitled "Process and Apparatus for Filtering Fluids".
The apparatus and method of that patent utilizes a precoat filtration method, wherein a precoat layer is formed on the filter elements and the fluid to be filtered enters the vessel and exits through the filter elements, passing through the filter aid cake deposited on the exterior of the filter elements. It has been observed that as the filtration process continues, "bridging" of the cake deposited on the exterior of the filter elements may occur. In this case, the cake surrounding individual elements within a filter vessel becomes so thick, that it joins the cake which has formed on the exterior of an adjacent filter element.
When such bridging occurs, large forces are generated across the filter elements which cause element distortion or bending. Any lateral distortion of a spiral wedge-wire filter element causes the wire spacing to increase on one side, thus permitting filter cake material to exit the filter and be present in the filtrate outlet. Fracture of the elements by bridging is also possible. With other type of filter elements, wherein a plurality of apertures are formed in the body of the filter element, or other type of filter elements, the damage may demonstrate itself in breakage of the element.
Another undesirable effect has been noted, wherein one of the filter elements, for any reason, becomes damaged, and the filtrate exiting the filter element contains undesirable contaminants and solid particles which under normal circumstances are filtered out by the filter element. Once a suspended solid leakage is detected at the outlet, it is a usual practice to discontinue filtering process and check oil filter elements within the filtration vessel for leakage. This requires time and labor and adversely effects productivity of a particular filtration vessel.
Still, a further problem has been recognized, wherein during a dry cake discharge process, a certain amount of liquid is left in the filtration vessel, when pressurized air introduced into the filtration vessel forces the water remained in the filtration vessel outside of the vessel through the filter element but has no sufficient strength to force entire quantity of water from the vessel. A certain amount of water or liquid is left in the vessel within the lower portion of the filtration element, such that at least part of the coating on the filter element remains saturated with liquid and cannot be disposed of with dry cake discharge method and also this fluid mixes with the dried cake during cake discharge. If the air flow is interrupted, the cake deposited on the exterior of the lower portion of the filter element simply falls under gravity to the bottom of the vessel and cannot be properly dried prior to discharge.
It is, therefore, an object of the present invention to provide an improved apparatus and method for filtering fluids which overcomes deficiencies of the prior art.
It is another object of the present invention to provide an improved apparatus for filtering fluids, wherein each filter element within a filter vessel can be checked for malfunctioning and isolated from the process, if desired.
It is still a further object of the present invention to provide an improved filter apparatus and method, wherein substantially all liquid from the filtration vessel is removed, allowing a virtually dry cake discharge. These and other objects of the present invention will be more apparent to those skilled in the art from the following description of the invention.